Steam? Even without your game being available to buy on Steam, they still have an API that you can hook into to do many of the same sorts of things that GameCenter does right? Achievements and the like.

You might want to look into Kongregate. Their API is only officially distributed as Javascript and ActionScript, but it wouldn't be too difficult to interact through a webview or something similar (or potentially rewrite the library to be native from your language of choice, I haven't really looked into how open their library is).

As far as Skorche's Steamworks suggestion, you have to apply and be approved for it, and I don't believe that it's something that's just handed out to anybody and everybody. Still, it is free, and it's not like it would cause any sort of harm to try.